I once sent him a letter (snail mail) addressed to Donald Knuth, c/o Stanford Computer Science Department (address at bottom of http://www-cs.stanford.edu/ http

I once sent him a letter (snail mail) addressed to Donald Knuth, c/o Stanford Computer Science Department (address at bottom of http://www-cs.stanford.edu/ http://www-cs.stanford.edu/). He got back within a couple of weeks, so it seems to have reached him fairly efficiently.